Surge protection circuit

ABSTRACT

A surge protection circuit for a circuit having a rectification module. The surge protection circuit includes a first diode, a second diode, a capacitor and a discharge device. The anode of the first diode is connected to a first input of the rectification module, and the anode of the second diode is connected to a second input of the rectification module. The cathodes of the first and second diodes are both connected to the first plate of the capacitor. The second plate of the capacitor is connected to the negative output of the rectification module. The capacitor is configured such that it is consistently charged to substantially the peak value of a supply voltage during normal operation between surge events. The discharge device is connected to the first plate of the capacitor and is configured to discharge the capacitor when the voltage across the capacitor is in excess of the peak of the maximum value of the normal supply voltage and not discharge the capacitor when the voltage across the capacitor is not in excess of the peak of the maximum value of the normal supply voltage.

This application claims the priority under 35 U.S.C. §119 of Europeanpatent application no. 09252933.8, filed on Dec. 30, 2009, the contentsof which are incorporated by reference herein.

FIELD OF THE INVENTION

The present disclosure relates to the field of surge protectioncircuits, in particular, but not exclusively to surge protectioncircuits for circuits comprising a rectification module.

BACKGROUND OF THE INVENTION

There is a need to protect circuits, particularly circuits having abridge rectifier module and a power factor correction (PFC) module fromsurge events/impulses in a power supply. Such surge events may be causedby lightning strikes or increases in a mains supply, for example.

Known ways of protecting against surge events include using at least twoelements: a varistor between mains connectors and the bridge rectifier;and a bulk capacitor either directly behind the bridge rectifier or inparallel across the outputs of a PFC module. Further details of suchsurge protection is provided below in relation to FIG. 2.

One or more embodiments of the invention described herein can provide animproved surge protection circuit over the prior art, particularly forprior art circuits where there is no current path between the inputvoltage and a bulk capacitor. The improvement can lie in the use offewer, and possibly the need for lower quality/specification, componentsthan the prior art in order to provide adequate surge protection.

The listing or discussion of a prior-published document or anybackground in this specification should not necessarily be taken as anacknowledgement that the document or background is part of the state ofthe art or is common general knowledge.

According to a first aspect of the invention, there is provided a surgeprotection circuit for a circuit comprising a rectification module, thesurge protection circuit comprising:

a first diode;

a second diode;

a capacitor; and

a discharge device; wherein

a first terminal of the first diode is connectable to a first input ofthe rectification module, a first terminal of the second diode isconnectable to a second input of the rectification module, secondterminals of the first and second diodes are both connected to the firstplate of the capacitor, and the second plate of the capacitor isconnectable to one of the outputs of the rectification module; andwherein:

the capacitor is configured such that it is consistently charged tosubstantially a peak value of a supply voltage during normal operation;and

the discharge device is connected to the capacitor and is configured todischarge the capacitor when the voltage across the capacitor is inexcess of the peak value of the maximum supply voltage and notsubstantially discharge the capacitor when the voltage across thecapacitor is not in excess of the peak value of the supply voltageduring normal operation.

In some examples, the first and second diodes could be provided as partof a further bridge rectifier provided in parallel with therectification module or as separate discrete components. It will beappreciated that the provision of the first and second diodes does notpreclude the option of further diodes also being provided, for examplethe provision of a third and fourth diode as part of a further bridgerectifier.

The first terminals of the diodes may be the anode of the diodes, andthe second terminals of the diodes may be the cathodes, or vice versa.The orientation of the diodes can be selected so as to provide a closedcircuit for charging the capacitor from the mains supply, whereby theclosed circuit includes a diode of the rectification module connected toone of the plates of the capacitor, and the first and second diodes areconnected to the other plate of the capacitor.

There may be provided a surge protection circuit for a circuitcomprising a rectification module, the surge protection circuitcomprising:

a first diode;

a second diode;

a capacitor; and

a discharge device; wherein

the anode of the first diode is connectable to a first input of therectification module, the anode of the second diode is connectable to asecond input of the rectification module, the cathodes of the first andsecond diodes are both connected to the first plate of the capacitor,and the second plate of the capacitor is connectable to the negativeoutput of the rectification module; and wherein:

the capacitor is configured such that it is consistently charged tosubstantially a peak value of a supply voltage during normal operation;and

the discharge device is connected to the first plate of the capacitorand is configured to discharge the capacitor when the voltage across thecapacitor is in excess of the peak value of the maximum supply voltageand not substantially discharge the capacitor when the voltage acrossthe capacitor is not in excess of the peak value of the supply voltageduring normal operation.

There may be provided a surge protection circuit for a circuitcomprising a rectification module, the surge protection circuitcomprising:

a first diode;

a second diode;

a capacitor; and

a discharge device; wherein

the cathode of the first diode is connectable to a first input of therectification module, the cathode of the second diode is connectable toa second input of the rectification module, the anodes of the first andsecond diodes are both connected to the first plate of the capacitor,and the second plate of the capacitor is connectable to the positiveoutput of the rectification module; and wherein:

the capacitor is configured such that it is consistently charged tosubstantially a peak value of a supply voltage during normal operation;and

the discharge device is connected to the second plate of the capacitorand is configured to discharge the capacitor when the voltage across thecapacitor is in excess of the peak value of the maximum supply voltageand not substantially discharge the capacitor when the voltage acrossthe capacitor is not in excess of the peak value of the supply voltageduring normal operation.

SUMMARY OF THE INVENTION

According to a further aspect of the invention, there is provided asurge protection circuit for a circuit comprising a rectificationmodule, the surge protection circuit comprising:

a first diode;

a capacitor; and

a discharge device; wherein

a first terminal of the first diode is connectable to an output of therectification module, a second terminal of the first diode is connectedto the first plate of the capacitor, and the second plate of thecapacitor is connectable to the other output of the rectificationmodule; and wherein:

the capacitor is configured such that it is consistently charged tosubstantially a peak value of a supply voltage during normal operation;and

the discharge device is connected to the capacitor and is configured todischarge the capacitor when the voltage across the capacitor is inexcess of the peak value of the maximum supply voltage and notsubstantially discharge the capacitor when the voltage across thecapacitor is not in excess of the peak value of the supply voltageduring normal operation.

There may be provided a surge protection circuit for a circuitcomprising a rectification module, the surge protection circuitcomprising:

a first diode;

a capacitor; and

a discharge device; wherein

the anode of the first diode is connectable to the positive output ofthe rectification module, the cathode of the first diode is connected tothe first plate of the capacitor, and the second plate of the capacitoris connectable to the negative output of the rectification module; andwherein:

the capacitor is configured such that it is consistently charged tosubstantially a peak value of a supply voltage during normal operation;and

the discharge device is connected to the first plate of the capacitorand is configured to discharge the capacitor when the voltage across thecapacitor is in excess of the peak value of the maximum supply voltageand not substantially discharge the capacitor when the voltage acrossthe capacitor is not in excess of the peak value of the supply voltageduring normal operation.

There may be provided a surge protection circuit for a circuitcomprising a rectification module, the surge protection circuitcomprising:

a first diode;

a capacitor; and

a discharge device; wherein

the cathode of the first diode is connectable to the negative output ofthe rectification module, the anode of the first diode is connected tothe first plate of the capacitor, and the second plate of the capacitoris connectable to the positive output of the rectification module; andwherein:

the capacitor is configured such that it is consistently charged tosubstantially a peak value of a supply voltage during normal operation;and

the discharge device is connected to the second plate of the capacitorand is configured to discharge the capacitor when the voltage across thecapacitor is in excess of the peak value of the maximum supply voltageand not substantially discharge the capacitor when the voltage acrossthe capacitor is not in excess of the peak value of the supply voltageduring normal operation.

The discharge device may not substantially draw any current from a mainssupply or the capacitor during normal operation. Although a smallleakage current may be present that causes the capacitor to discharge bya very small amount during normal operation, this may not be consideredas “significantly” discharging the capacitor to a value below the peakmains supply voltage.

In this way, the capacitor is only discharged following a surge eventand not for every half cycle of the received mains supply as is the casewith the prior art. Embodiments of the invention enable a lowspecification, low cost capacitor to be used as its charge is onlysubstantially changed when a surge event occurs and not during “normal”operation, that is, when a surge event has not occurred. Morespecifically, the capacitor can only be charged in excess of the peakmains supply voltage when a surge event occurs, and may only bedischarged back to the peak mains supply voltage, and not significantlybelow the peak mains supply voltage, when the surge event finishes.Normal operation may be considered as times at which surge events arenot occurring and/or a mains dip is not occurring, and when the effectsof a surge event and/or mains dip are not present in the circuit.

In this way, the lifetime of the capacitor can be extended compared withexamples where a capacitor is charged and discharged for everyhalf-cycle of a mains supply voltage.

Similarly, as the charge on the capacitor is not substantially changingduring normal operation of the circuit, current may not substantiallyflow through the diodes during normal operation either. Therefore, lowquality/specification diodes may also be deemed acceptable.

The capacitor may be configured such that its charge increases when asurge event occurs, and the increased charge may mainly discharge to thedischarge device after the surge event. The increase in charge may beconsidered as the charge in excess of the steady-state charge suppliedby the mains supply voltage during normal operation between surgeevents.

One or more embodiments described herein can enable a robust, economicalsurge protection circuit to be provided, and the surge protectioncircuit may be particularly suitable for circuits comprising arectification module, a power factor correction (PFC) module and aswitched mode power supply, for example. Switch-mode power supplies thatdo not have a direct current path to the output capacitor mayparticularly benefit from embodiments of the invention, as such powersupplies may have to be protected from surges in a different way to thatknown from the prior art.

The circuit may further comprise a power factor correction (PFC) module,such as an isolated or a non-isolated PFC. The two outputs of therectification module may be connected as inputs to the power factorcorrection module. The PFC may be a switched mode power supply (SMPS).The surge protection circuit may be configured to protect the PFC/SMPSfrom surge events/impulses in a mains supply.

The discharge device may be considered as comprising an activationcomponent that automatically “activates” a dissipation component thatdissipates energy only when a surge event occurs. Such an activationcomponent may be a Zener diode or a transistor with a voltage divider orwith comparator. A resistor is an example of a component that dissipatesenergy.

The discharge device may comprise a discharge resistor and a Zenerdiode. The Zener diode may be considered as an example of an “activationcomponent”, and the resistor may be considered as an example of anenergy dissipating component that is provided with energy from a surgeevent when the activation component is activated.

The discharge resistor and Zener diode are examples of components thatcan form part of the discharge device. It will be appreciated that thesecomponents can be used such that they will not consume any energy, andtherefore not unnecessarily discharge the capacitor, unless a surgeevent occurs. In this example, the discharge device may act as an opencircuit during normal operation of the circuit as the breakthroughvoltage of the Zener diode will not be exceeded.

In one embodiment the discharge resistor and Zener diode may beconnected in series between the first plate of the capacitor and thesecond plate of the capacitor. A first pin of the discharge resistor maybe connected to the first plate of the capacitor, a second pin of thedischarge resistor may be connected to the cathode of the Zener diode,and the anode of the Zener diode may be connected to the second plate ofthe capacitor. In other examples, the relative positions of thedischarge resistor and Zener diode can be reversed. In this way, thecapacitor may discharge the charge of the surge event via the resistorwhen the breakthrough voltage of the Zener diode is exceeded.

In another embodiment the discharge resistor and Zener diode may beconnected in series between the first plate of the capacitor and anoutput of the power factor correction module. A first pin of thedischarge resistor may be connected to the first plate of the capacitor,a second pin of the discharge resistor may be connected to the cathodeof the Zener diode, and the anode of the Zener diode may be connectedindirectly to the second plate of the capacitor via an output of anon-isolated PFC module. In this way, the capacitor may discharge thecharge of the surge event to the resistor and the output of the PFCmodule when the breakthrough voltage of the Zener diode is exceeded.

The Zener diode may have a breakthrough voltage that is higher than anexpected peak voltage during normal operation. In embodiments where theZener diode and the resistor are connected in series directly with thesecond plate of the capacitor, and so to the low output of therectification module, the expected peak voltage during normal operationcan represent the maximum operating voltage of the mains supply. Inembodiments where the Zener diode and the resistor are connected to anoutput of the PFC module, and therefore indirectly to the second plateof the capacitor, the expected peak voltage during normal operation canrepresent the difference between the maximum operating voltage of themains supply and the minimum output voltage of the PFC module (includinga ripple voltage component where appropriate).

In this way, the Zener diode only conducts, and therefore allows theresistor to dissipate energy, when the charge on the capacitor is inexcess of charge that represents the expected peak voltage across thecomponent (which can depend upon how the capacitor is connected in thecircuit) during normal operation.

The breakthrough voltage may be of the order of 5% or 10% higher thanthe expected peak voltage. For example, for a mains supply voltage witha peak value of 375 volts the breakthrough voltage may be greater than380 volts. In some examples, a surge event may represent a voltage inthe range of 0.5 kV to 4 kV depending upon its severity level.

In some examples, the capacitor is only discharged when the voltageacross the capacitor exceeds the breakthrough voltage of the Zenerdiode. The capacitor will not discharge when the voltage does not exceedthe breakthrough voltage of the Zener diode, as the Zener diode will notallow a current to flow through it; this is indicative of “normal”operation of the power supply.

It will also be appreciated that other components can be used as thedischarge device that operate in the same way such that they can onlyact as a load when a surge event occurs. For example, a variety ofactive components such as bipolar transistors, MOSFET's, etc., having aconduction state that is changeable from on to off by means of a veryhigh voltage divider in parallel to the capacitor can be used as anactivation component. In this way, the discharge device is activated todischarge the capacitor when an expected peak voltage is exceeded andthe threshold voltage of the transistor is satisfied so that the currentchannel of the transistor conducts. Such a transistor may be provided inseries with a discharge resistor.

There may be provided a switched mode power supply comprising any surgeprotection circuit disclosed herein.

There may be provided an electronic device comprising any surgeprotection circuit disclosed herein.

According to a further aspect of the invention, there is provided amethod of operating a surge protection circuit for a circuit comprisinga rectification module, the surge protection circuit comprising:

a first diode;

a second diode;

a capacitor; and

a discharge device; wherein

a first terminal of the first diode is connected to a first input of therectification module, a first terminal of the second diode is connectedto a second input of the rectification module, a second terminal of thefirst and second diodes are connected to the first plate of thecapacitor, and the second plate of the capacitor is connected to one ofthe outputs of the rectification module; and wherein:

the discharge device is connected to the capacitor;

the method comprising:

during normal operation:

-   -   keeping the capacitor consistently charged to substantially a        peak value of a supply voltage; and

in case of a surge event:

-   -   increasing the charge on the capacitor with current received as        part of the surge event;    -   discharging the current to the discharge device after the surge        event.

The increase in charge on the capacitor may be considered as chargecurrent in excess of that associated with the normal peak operatingvoltage of the mains supply.

According to a further aspect of the invention, there is provided amethod of operating a surge protection circuit for a circuit comprisinga rectification module, the surge protection circuit comprising:

a first diode;

a capacitor; and

a discharge device; wherein

a first terminal of the first diode is connected to an output of therectification module, a second terminal of the first diode is connectedto the first plate of the capacitor, and the second plate of thecapacitor is connected to the other output of the rectification module;and wherein:

the discharge device is connected to the capacitor;

the method comprising:

during normal operation:

-   -   keeping the capacitor consistently charged to substantially a        peak value of a supply voltage; and

in case of a surge event:

-   -   increasing the charge on the capacitor with current received as        part of the surge event;    -   discharging the current to the discharge device after the surge        event.

BRIEF DESCRIPTION OF THE DRAWINGS

A description is now given, by way of example only, with reference tothe accompanying drawings, in which:

FIG. 1 illustrates a block diagram of a prior art power factorcorrection circuit (PFC);

FIG. 2 illustrates a block diagram of a prior art active boost PFCstage;

FIG. 3 illustrates a prior art isolated active flyback PFC stage;

FIGS. 4 a to 4 d illustrate surge protection circuits according toembodiments of the invention;

FIG. 5 illustrates a surge protection circuit according to anotherembodiment of the invention;

FIG. 6 illustrates a surge protection circuit according to anotherembodiment of the invention;

FIG. 7 illustrates a surge protection circuit according to anotherembodiment of the invention

FIG. 8 illustrates a surge protection circuit according to anotherembodiment of the invention; and

FIG. 9 illustrates graphically use of a surge protection circuitaccording to an embodiment of the invention;

FIG. 10 illustrates a surge protection circuit according to anotherembodiment of the invention; and

FIG. 11 illustrates a surge protection circuit according to anotherembodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

One or more embodiments described herein can provide a surge protectioncircuit that comprises a capacitor that is charged by a surge event, anda discharge device to discharge the capacitor following the surge event.The capacitor may be discharged after a surge event until the chargeacross the capacitor is substantially constant, for example at a levelthat corresponds to an expected peak value of the maximum (mains) supplyvoltage across the capacitor during normal operation. In this way, a lowquality/specification capacitor can be used as the charge across thecapacitor only significantly changes during and after a surge event.When a surge event is not occurring, the charge across the capacitor maybe substantially constant. This can be in contrast to the prior artwherein any capacitor that is located at a similar position to thecapacitor of the invention charges and discharges every half cycle of amains input alternating current (AC) voltage. In such prior artexamples, the capacitor needs to be of a minimum quality, which ishigher than the minimum quality required by an embodiment of theinvention, in order to be capable of transferring requisite amounts ofenergy to the output during normal operation. Furthermore, a capacitorof the prior art may completely or significantly discharge following asurge event and not be kept at a substantially constant value.

A capacitor that may be considered as suitable for embodiments of thepresent invention may be chosen to satisfy the requirements of thevoltage associated with a surge event, and not necessarily fortransferring power to the output. For example, the properties of acapacitor in terms of rms-current may not need to be accounted foraccording to embodiments of the invention.

For mains driven applications with high and medium power rates,switch-mode power supplies are often used in order to increase theefficiency of the application. Such switch-mode power supplies are alsoused to fulfil requirements concerning the necessity of nearlysinusoidal input currents to satisfy a standard, such as Europeanstandard EN61000-3-2. Thus an active power factor correction (PFC)circuit is often used as a primary stage followed by a bulk capacitor tosmooth the output voltage and generate an output voltage that is asconstant as possible (within economical justifiable limits) forsubsequent dc-dc converters. FIG. 1 illustrates a block diagram of amains driven power supply with power factor correction.

A problem with such an active PFC stage can be the immunity of the mainsdriven application to surge events. A surge event can be caused by alightning strike, or can be a surge directly from the mains supplier,for example by the switching actions of external power systems or load.

In order not to be damaged irreparably, mains driven power convertersshould be protected against surge impulses. These surge impulses cancause failures in sensitive electronic devices as well as in more ruggedpower components such as the power switch of switch-mode power supplies.Therefore, such mains driven applications should be protectedeffectively against surge events. In some examples, a varistor can beused as a first protection element, but the varistor may not besufficient to completely protect the power converter.

In many cases the PFC-stage depicted in FIG. 1 is realized by a boostconverter. FIG. 2 shows a circuit diagram of a typical boost converter200.

The boost converter of FIG. 2 is an example of an SMPS where there is adirect, permanent, current path from the input to the capacitor 202. Avaristor 204 along with the large bulk electrolytic capacitor 202 (forexample of the order of greater than 100 μF in case of a 2 kV surgeseverity level) may be considered to adequately protect an SMPS.However, if a varistor with a medium sized bulk electrolytic capacitor(for example smaller than 100 μF, for example of the order of 68 μF) isused for power transfer purposes (or for any other reason), thenadequate surge protection may not be provided.

In particular using a bulk output capacitor 202 for surge protection maynot be sufficient to efficiently defend the switch-mode power supplyfrom any damage, especially in low power applications where only smallcapacitance values are required for standard operation. It will beappreciated from the description that follows that embodiments of theinvention can provide improved protection for boost converters.

A VDR 204 (voltage dependent resistor) is also provided in parallelacross the input voltage supply to provide surge protection. The VDR maybe a varistor. It will be appreciated that the HF-Filter of the circuitshown in FIG. 2 has chokes/inductors in series along the input currentpath. The HF-filter can provide some series impedance which limits (tosome degree) the current to the output capacitor.

In examples where there is a no direct, permanent, current path from theinput to a bulk capacitor, then a known varistor may not protect theSMPS effectively, as the varistor may not be sufficient to protect fromsurge voltages of up to 800V to 1500V. Therefore, additional protectiondevices/components may be needed.

In some prior art examples, a galvanic isolation is used within themains driven power supply to improve the safety of the circuit. It canbe possible to realize this galvanic isolation in the dc-dc stage or itmay be provided in the primary PFC-stage. Realising this galvanicisolation in the primary PFC stage can be useful for low and mediumpower ranges where a flyback-converter can be used instead of a boostconverter. FIG. 3 depicts a circuit diagram of a flyback converter 300.

In contrast to the boost PFC stage, an emerging surge impulse will findno path to charge the bulk output capacitor 302 and instead charges theparasitic capacitance at the primary side of the transformer responsiblefor galvanic isolation (for example, the parasitic capacitance of themain switch 304 of the PFC 300). Any small capacitance that is providedby an HF-filter capacitor may not be sufficient to suppress the surge incombination with the VDR 306. The PFC 300 can be destroyed as aconsequence. Hence, the usage of a mains driven flyback-converter 300often requires further protection devices.

The way in which surge protection is provided by the prior art can bedifferent depending upon the configuration of the PFC stage that isbeing used.

Other PFC stages that do not provide a direct, permanent, current pathfrom the line input to the bulk electrolytic output capacitor are known,and include non-isolated PFC-converters like PFC-Buck or PFC-Buck-Boostconverters or Cuk, Sepic etc. PFC-topologies. In such examples, there iseither no direct path available (flyback, forward, Cuk, Sepicconverters) or the direct path is via a switch which could be open (Buckand Buck-Boost converters). As no current can flow into the outputcapacitor with these topologies, nearly all of the current will flowthrough the varistor, which implicates a high voltage level that isapplied to the switch-mode power supply (SMPS). Such a high voltage candestroy components of the SMPS. Therefore, use of a single varistor forsurge protection of such topologies may not be sufficient, andadditional components like several varistors or a combination of avaristor and Transil-Diode are employed in the prior art. However,additional varistors or Transils can be very expensive.

One or more embodiments of the invention that provide an additionalcapacitor in parallel with the SMPS can provide a second path for asurge current, and can therefore reduce the likelihood of the SMPS beingexposed to potentially damaging high voltages.

A feature of one or more embodiments of the invention presented here isto use only one additional bulk electrolytic capacitor and only one ormore slow rectifying diodes for surge protection in order to take up amajor part of the energy in case of a surge event. In addition, adischarge device can be used so that the additional bulk electrolyticcapacitor is only charged and discharged when a surge event occurs andnot during “normal” operation. The discharge device may be selected sothat it can sufficiently discharge from a previous surge event before anext surge event occurs. The repetition rate and the severity level ofthe surge events may be defined by the IEC 61000-4.5 and IEEE C61.41.2standard.

If a surge protection circuit according to an embodiment of theinvention is not used, then the energy of a surge impulse coupled to oneor both mains input voltage lines will be directly transferred to thePFC stage and will eventually destroy some of the components of the PFCstage. Inserting an additional bulk electrolytic capacitor along withthe discharge device according to embodiments of the present inventioncan improve the situation.

FIG. 4 a illustrates a surge protection circuit according to anembodiment of the invention. In comparison with the prior art, twodiodes D₁ and D₂ 420, 422 as well as a capacitor C₂ 424 are provided.The anode of the first diode D₁ 420 is connected to a first mains powerline 428, and the anode of the second diode D₂ 422 is connected to asecond mains power line 430. It will be appreciated that a surge eventof any polarity can be experienced on both the first and second mainspower lines 428, 430 as mains power is AC. The diodes D₁ 420 and D₂ 422together with capacitor 424 may provide sufficient surge protection fora surge impulse of any polarity in combination with any polarity of themains.

The cathode of both the first diode D₁ 420 and second diode D₂ areconnected to a first plate of the capacitor C₂ 424.

The second plate of the capacitor C₂ 424 is connected to one of theoutputs of the rectification module 433, in this example, the low outputof the rectification module 433.

Also, the first plate of the capacitor C₂ 424 is connected to adischarge device 426, examples of which are described below.

When a surge impulse is received at the mains power lines 428, 430, amajor part of the energy will charge the additional capacitor C₂ 424 viaone of the additional diodes D₁ and D₂ 420, 422. Prior to the occurrenceof the surge impulse, the capacitor 424 is charged to the peak of thesinusoidal input voltage.

It will be appreciated that the voltage value across the additionalcapacitor C₂ 424 after a surge event depends on its capacitance valueand it has been found that the usage of a bulk electrolytic capacitor asC₂ will keep the voltage level applied to the PFC stage 432 within anacceptable range of voltage values, thereby protecting the PFC stage432.

In order to comply with any standards that may be in place, the deviceunder test may have to withstand several successive surge impulses.Therefore, it may be ensured that the additional capacitor C₂ 424 candischarge to the original voltage level before the next surge impulse isexpected in order to be able to effectively absorb the energy of thenext surge impulse. The repetion rate of surge events and the severitylevel can vary for different applications, and therefore the capacitanceof the additional capacitor C₂ can be selected accordingly.

FIG. 4 b illustrates a surge protection circuit that is similar to thatof FIG. 4 a. In FIG. 4 b, only one diode D₁ 420′ is present. The anodeof the diode D₁ 420′ is connected to the positive dc output of therectification module 433′, and the cathode of the diode D₁ 420′ isconnected to the first plate of the capacitor C₂ 424.

It will be appreciated that the surge protection circuit of FIG. 4 bonly requires a single diode because it is configured to process a surgeevent after rectification, and therefore the surge event will only beexperienced by the DC output of the rectification module 433′.Nonetheless, the surge protection circuit of FIG. 4 b can still serve toprotect the PFC stage 432′.

FIG. 4 c illustrates a surge protection circuit according to anembodiment of the invention. In this example, the cathodes of the firstand second diodes 420″, 422″ are connected to the two inputs to therectification module, and the anodes of the first and second diodes420″, 422″ are connected to the first plate of the capacitor 424″. Thesecond plate of the capacitor 424″ is connected to the positive outputof the rectification module. The discharge device 426″ is connected tothe second plate of the capacitor 424″, which in this example is thepositive plate of the capacitor during operation.

It will be appreciated that the embodiment of FIG. 4 c is similar tothat of FIG. 4 a. The closed circuit that is provided to charge thecapacitor 424″ of FIG. 4 c uses the additional diodes 420″, 422″ forproviding a current path between the mains supply and the negative plateof the capacitor 424″, and diodes of the rectification module forproviding a current path between the mains supply and the positive plateof the capacitor 424″.

In contrast, the closed circuit that is provided to charge the capacitor424 of FIG. 4 a uses the additional diodes 420, 422 for providing acurrent path between the mains supply and the positive plate of thecapacitor 424, and diodes of the rectification module for providing acurrent path between the mains supply and the negative plate of thecapacitor 424.

FIG. 4 d illustrates a surge protection circuit according to anembodiment of the invention. In this example, the cathode of the firstdiode 420′″ is connected to the negative output of the rectificationmodule, and the anode of the first diodes 420′ is connected to the firstplate of the capacitor 424′″. The second plate of the capacitor 424′″ isconnected to the positive output of the rectification module. Thedischarge device 426″ is connected to the second plate of the capacitor424′″, which in this example is the positive plate of the capacitorduring operation.

It will be appreciated that the embodiment of FIG. 4 d is similar tothat of FIG. 4 b. The closed circuit that is provided to charge thecapacitor 424′″ of FIG. 4 d uses the additional diode 420′ and a diodeof the rectification module for providing a current path between themains supply and the negative plate of the capacitor 424′″, and a diodeof the rectification module for providing a current path between themains supply and the positive plate of the capacitor 424′″.

In contrast, the closed circuit that is provided to charge the capacitor424′ of FIG. 4 b uses the additional diode 420′ and a diode of therectification module for providing a current path between the mainssupply and the positive plate of the capacitor 424′, and a diode of therectification module for providing a current path between the mainssupply and the negative plate of the capacitor 424′.

Examples of discharge device 426 according to embodiments of theinvention will be described with reference to at least FIGS. 5 and 6.Although the embodiments that follow are based on a surge protectioncircuit with the structure of FIG. 4 a, it will be appreciated that theembodiments are equally applicable to a surge protection circuit withthe structure of any of FIG. 4 b, 4 c or 4 d.

FIG. 5 illustrates an embodiment wherein the circuit depicted in FIG. 4a has been extended by a resistor R_(discharge) 534 and a Zener diodeD_(Zener) 536 with a breakthrough voltage level just above the peak ofthe maximum operation voltage. In this example, a first pin of theresistor R_(discharge) 534 is connected to the first plate of thecapacitor 524. The second pin of the resistor 534 is connected to thecathode of the Zener diode D_(Zener) 536. The anode of the Zener diodeD_(Zener) 536 is connected to the second plate of the capacitor 524.

The passive discharge device of FIG. 5 (comprising the resistorR_(discharge) 534 and Zener diode D_(Zener) 536) provides for dischargethrough the resistor R_(discharge) 534 when a surge event occurs, thatis, when the breakthrough voltage of the Zener diode D_(Zener) 536 isexceeded. The Zener diode 536 may be considered as an activationcomponent as it only activates the discharge device when itsbreakthrough voltage is exceeded. The resistor 534 may be considered asa current limiting component. In some examples, most of the dissipationcan be performed by the Zener diode 536.

It will be appreciated that the locations of the discharge resistorR_(discharge) 534 and Zener diode 536 may be exchanged, and that thedischarge resistor R_(discharge) 534 and Zener diode in series in anyconfiguration can perform the required functionality of only dischargingC₂ 524 when a surge event occurs. The Zener diode can consist of aseries connection of two or several Zener diodes with lower breakdownvoltage. For example, a 400V-Zener diode can be implemented by a seriesconnection of two 200V Zener diodes or four 100V-Zener diodes.

According to another embodiment, the energy of the surge impulse can betransferred to the output of the PFC-stage. Such an embodiment isillustrated as FIG. 6. The difference between the circuits of FIGS. 5and 6 is that the anode of the Zener diode D_(Zener) 536 is connected tothe high voltage output of the PFC stage 632 instead of the second plateof the capacitor 624. In this example, the discharge path to the secondplate of the capacitor 624 is completed by the output capacitor (C) andthrough the PFC 632.

For the embodiment of FIG. 5, the capacitor 524 will discharge viaR_(discharge) 534 when the voltage of the additional capacitor C₂ 524 ishigher than the breakthrough voltage of the Zener diode. Thebreakthrough voltage of the Zener diode may be higher than the peakvoltage of the maximum operating voltage of the mains supply.

For the embodiment of FIG. 6, the capacitor 624 will discharge viaR_(discharge) 634 when the voltage of the additional capacitor C₂ 624 ishigher than the sum of the output voltage and breakthrough voltage ofthe Zener diode. The breakthrough voltage of the Zener diode may beselected so that it is higher than the difference of the peak voltage ofthe maximum operating voltage of the mains supply and the minimum PFCoutput voltage, assuming that the output voltage is lower than the peakvoltage of the mains supply.

The embodiment of FIG. 6 may be particularly suitable for non-isolatedPFC stages as a discharge path via the PFC stage 632 is available,whereas the embodiment of FIG. 5 may be suitable for both isolated andnon-isolated PFC stages.

With reference to FIG. 5, the discharge device is configured so that thecapacitor 524 is not discharged when the voltage across the capacitor524 is below the peak value of the supply voltage during normaloperation, and an example will now be described with reference to amains voltage supply in Europe. The input voltage range is 180 to 264VAC, rms. In order not to discharge at high mains, the Zener diode 536voltage can be chosen equal or higher than sqrt(2)*264V=375V. Thismeans, that if during operation at nominal voltage 230V a surge eventoccurs, the capacitor 524 voltage will rise to for example 450V. Thus,the Zener diode 536 will conduct so as to discharge the capacitor 524via the resistor 534. This discharge will stop around 375V when theZener diode 536 no longer conducts. However, afterwards the dischargewill continue to sqrt(2)*230V=325V due to the unavoidable small leakagecurrent of the capacitor 524 (this last discharge might even takeminutes). Thus, under normal operation, the capacitor 524 will becharged to the peak of the momentary AC voltage (325V in our example of230V AC, rms).

In examples where the discharge device is connected to the second plateof the capacitor such as that shown in FIG. 5, the breakthrough voltageof the Zener diode 536 should be greater than the peak of the maximummains voltage, so greater than 375V.

In examples where the discharge device is connected to the output of thePFC module, such as that illustrated in FIG. 6, a Zener diode 636 havinganother breakthrough voltage should be used. Taking a non-isolatedPFC-Buck converter as an example, it could have an output voltage of100V+ripple. So let's assume the minimum output voltage is 75V. In caseof the maximum input voltage of 265V, rms the capacitor 624 is again at375V during normal operation. However, the discharge device is connectedto the output of the PFC stage, thus to a minimum voltage level of 75V.Hence, the voltage applied on the discharge device is only 300V. In suchan example, a Zener diode 636 with a breakthrough voltage of greaterthan 300V would be sufficient.

Embodiments of the invention can provide advantages for isolated PFC's,where there is no electrolytic capacitor in direct connection to themains. The big electrolytic capacitor is at the secondary side of themains isolating transformer for isolated PFC's and therefore may not beavailable for surge protection. The additional surge protection providedby embodiments of the invention can prevent damage to the PFC.

Embodiments of the invention can also be used with a non-isolated boostPFC that has very small output powers, whereby the electrolyticcapacitor that is used to limit the 100 Hz-ripple is too small to absorbthe surge energy.

For other non-isolated PFC's such as buck, buck-boost, Cuk, Sepic, theinvention can show the same benefit as that discussed with reference toisolated PFC's, since these topologies do not have a direct connectionbetween the supply voltage and the output capacitor.

Another embodiment of the invention is shown as FIG. 7. In this example,the PFC stage is a flyback-stage 732 that offers no direct path for asurge current to the output capacitor 702 across the outputs of theflyback-stage 732. In such an example, the varistor 701 across the inputvoltage supply may not be sufficient to provide surge protection, andthe additional capacitor C₂ 724 of an embodiment of the invention mayprovide the additional surge protection that is required. It willappreciated that the additional capacitor 724 can also be used for otherisolated PFC stages such as forward, half-bridge, full-bridge orresonant converters and for other non-isolated PFC stages with no directcurrent path from the input to the bulk capacitor such as Buck,Buck-Boost, Sepic, Cuk etc.

A further embodiment of the invention is shown as FIG. 8. In thisexample, the PFC stage is a Boost-stage 832 that provides a direct pathfor a surge current to the output capacitor 802. In such an example, afurther surge protection means in addition to the known varistor 801 andoutput capacitor 802 may be necessary.

The further surge protection means may be required as the value of theoutput capacitor C 802 is normally selected so as to transfer power, andis not selected to absorb the very seldom occurring surge impulses.Therefore, and especially in case of an SMPS with small power ratings,the value of the output capacitor C 802 may be too small to adequatelyprovide surge protection. That is, even the combination of the varistor801 and the output capacitor 802 may not be able to protect the powersupply efficiently. Therefore, an additional capacitor C₂ 824 of anembodiment of the invention may be required to provide adequate surgeprotection.

In prior art examples, the value of the output capacitor C 802 can beselected in order to provide a “hold-up” time for the output signal.This may be in addition to a consideration of the power that is to betransferred to the output when selecting the value of the outputcapacitor 802. The energy stored in the output capacitor C 802 can thenbe used to maintain the operation of the subsequent dc-dc stage 803 forshort periods of time during which the mains voltage breaks down (mainsdips), and a specified period of time that the output capacitor C 802can contribute to the output is known as the hold-up time. As aconsequence, the output capacitor C 802 can have a minimum capacitanceproportional to the stored energy so that a required hold-up time can beguaranteed. Often, the required hold-up time, and not the power transferrequirement, is used to determine the size of the high quality outputcapacitor C 802.

The additional capacitor C₂ 824 according to the embodiment of FIG. 8,in combination with the discharge device 826 can also be used tocontribute to a required hold-up time: The additional capacitor C₂ 824is charged to the peak of the line voltage in standard/normal operation(that is, periods of time when there is no surge and no mains-dips). Theenergy stored in the additional capacitor C₂ 824 can now be transferredin case of a mains dip to the input or output of the PFC-stage, so thatthe energy can help to maintain the operation of power supply unit.

An advantage provided by the embodiment of FIG. 8 is that the originaloutput capacitor C 802 can be made smaller since it has to bedimensioned only for the power rating and not the “hold-up time”, as theadditional capacitor C₂ 824 can contribute to the hold-up time as wellas providing surge protection. The hold-up time requirement is nowaddressed primarily by C₂ 824. It will be appreciated that in suchembodiments, the additional capacitor C₂ 824 is not dischargingpermanently in normal operation, but in the case of a seldom occurringmains dip event. However, in this case it is naturally discharged belowthe peak of the line voltage.

Different discharge devices for discharging the capacitor are possibleaccording to embodiments of the invention. A suitable discharge devicemay be selected based on the expected severity level and repetition rateof the surge impulses. The severity of a surge impulse may be determinedby the maximum occurring voltage of the surge impulse, and four severitylevels from 0.5 kV to 4 kV are known. Examples of suitable dischargedevices are provided below.

In practice, in some cases the leakage of a very large capacitor can beused as the discharge device. Such a capacitor can absorb several surgeimpulses, and have a leakage current that is “large” enough tosufficiently discharge the capacitor before a new series of surgeimpulses starts.

FIG. 9 illustrates graphically the results of testing an embodiment ofthe invention wherein the leakage current of a single capacitor providesthe functionality of the discharge device. The horizontal axis in thegraph of FIG. 9 represents time, and the vertical axis representsvoltage across the capacitor as the discharge device. This example maybe considered as suitable for protecting against surges with lowerseverity levels.

FIG. 9 shows the application of ten pulses with a severity level of 0.5kV, and a repetition rate of 6 seconds (according to the standard, arepetition rate of only 60 seconds is required). The testing revealsthat the voltage across the capacitor remains within acceptable limits(Vmax=490V) by selecting an appropriate value for the capacitor. This isbecause the capacitor discharges a little bit due to the leakage currentwithin 6 seconds between surge events. FIG. 9 shows that the capacitorwill not discharge below the peak of the mains voltage, as the mainswould charge it up to the peak value every 10 ms) as shown in FIG. 9.

An alternative discharge device includes a resistor in parallel with acapacitor. The additional resistor may be required if the leakagecurrent is not sufficient to adequately discharge the capacitor beforethe occurrence of the next surge event. In such examples, the resistormay have a high resistance value and can be used to ensure that thecapacitor is discharged to the peak of the main voltage before a nextsurge impulse. This discharge device may lead to some additional lossesas the capacitor will always be discharged a little bit, even instandard operation.

Such a discharge device is know in the art in relation to stackedcapacitors in order to symmetrise the voltage across the stackedcapacitor, as the leakage current can vary from part to part, as well asdue to temperature. A benefit of a huge parallel resistor can be thepossibility to design with a known well defined leakage current.Nevertheless, the amount of leakage current (which may be approximatelyfive times the worst case leakage) is still so small, that it should notbe considered as significant discharge of the capacitor within one mainscycle. Thus, also with such a resistor, the capacitor would still becharged to substantially a maximum value of the mains supply voltageduring normal operation. An advantage to using the resistor in parallelwith the capacitor is that a greater range of severity levels of a surgeevent can be dealt with.

It will be appreciated that the above examples of discharge devices arenot limiting, and that any discharge device that can perform thefunctionality of only substantially charging and discharging a capacitorduring and after a surge event, whilst not discharging the capacitorduring “normal” operation”, can be provided. Examples of suitabledischarge devices can also include active discharge devices, forexample, through use of an extra SMPS to transfer the energy stored inthe capacitor to the output.

Examples of active discharge devices that can be used with embodimentsof the invention can include bipolar transistors, MOSFETs etc., whereinthe conduction state of the component is changed from off to on by meansof very high voltage divider in parallel to the capacitance. In thisway, the active discharge component can turn on and provide thenecessary discharge path when the voltage is in excess of the peakvoltage of the maximum mains. In this way, it can also be envisionedthat hysteresis can be added to the discharge device, such thatdischarge is kept active to reach more quickly a lower capacitor voltagethan without hysteresis. This can be especially beneficial forrepetitive surge impulses.

A person skilled in the art will appreciate that there are numerous waysto provide this functionality, for example by using a voltage divider tobring the gate of a MOSFET above threshold when a surge event occurs, orby using a voltage divider which feeds a comparator whose outputturns-on the active component when a surge event occurs.

In some examples, a power supply can be used as the discharge devicesuch that it is activated after a surge event or in case of mains dips,and is otherwise deactivated during normal operation. In suchembodiments, the capacitor will still be charged to substantially amaximum value of the mains supply voltage during normal operation.

As indicated above in relation to FIG. 8, a hold-up time requirement canbe addressed by a capacitor according to an embodiment of the invention.The capacitor can be used to provide an output voltage during “mainsdips”. FIG. 10 illustrates a diagram of such an embodiment. In thisembodiment the output capacitor C₁ 902 may not have to be dimensioned toprovide hold-functionality, and can be dimensioned in accordance withthe desired power rating.

The circuit of FIG. 10 can transfer the energy stored by capacitor C₂924 to the input of the PFC-stage by operating an active switch 950,which may by bipolar transistor, a MOSFET, a IGBT etc. when a mains dipis detected.

FIG. 11 shows a circuit according to a further embodiment of theinvention, where the switch 950 of FIG. 10 is replaced by a DC-DCconverter 1050, which is only activated upon the detection of a seldomoccurring mains-dip event.

One or more of the surge protection circuits described herein can befitted to any electronic device that is susceptible to surgeevents/impulses, especially mains driven devices/applications. Examplesof such electronic devices include television sets, personal computers,DVD players, satellite/cable decoder boxes, stereo systems, and anyother electronic entertainment device, as well as non-entertainmentelectronic devices. A possible application is a power supply forLCD-televisions, monitors etc., for example an LCD-TV with LEDbacklights where a mains-isolated PFC-stage can be required. Amains-isolated PFC-flyback-stage can benefit from surge protectionprovided by embodiments of the invention. Lighting ballasts and externalpower-supplies (such as adapters) can also be used with embodiments ofthe invention. However, it will be appreciated that embodiments of theinvention are not limited to any specific type of electronic device.

In some embodiments only a single capacitor and only two diodes may beused to provide surge protection, optionally in combination with avaristor and this can offer a reduction in cost over the prior art.Furthermore, it may be possible to use lower quality/specificationcomponents as they are only operated when a surge event occurs and notduring normal operation. For example an inexpensive bulk capacitor canbe used to provide adequate surge protection.

One or more embodiments of the invention can enable standards such asEN61000-3-2 to be satisfied economically and efficiently. In someexamples, the surge protection circuit does not influence the inputcurrent waveform so that EN61000-3-2 is still fulfilled, andadditionally any standards concerning surge protection can also beaddressed.

1. A surge protection circuit for a circuit having a rectificationmodule, the surge protection circuit comprising: a first diode; a seconddiode; a capacitor; and a discharge device; wherein a first terminal ofthe first diode is connectable to a first input of the rectificationmodule, a first terminal of the second diode is connectable to a secondinput of the rectification module, second terminals of the first andsecond diodes are both connected to a first plate of the capacitor, anda second plate of the capacitor is connectable to one of the outputs ofthe rectification module; and wherein: the capacitor is configured suchthat it is consistently charged to substantially a peak value of asupply voltage during normal operation between surge events; and thedischarge device is connected to the capacitor and is configured todischarge the capacitor when the voltage across the capacitor is inexcess of the peak value of the maximum supply voltage and notsubstantially discharge the capacitor when the voltage across thecapacitor is not in excess of the peak value of the maximum supplyvoltage
 2. The surge protection circuit of claim 1, wherein thecapacitor is configured such that its charge increases when a surgeevent occurs, and the increased charge discharges to the dischargedevice after the surge event.
 3. The surge protection circuit of claim1, wherein the capacitor is configured such that it is onlysubstantially discharged after a surge event.
 4. The surge protectioncircuit of claim 1, wherein the discharge device comprises: a dischargeresistor; and a Zener diode; wherein the discharge resistor and theZener diode are connected in series between the first plate of thecapacitor and the second plate of the capacitor.
 5. The surge protectioncircuit of claim 4, wherein the Zener diode has a breakthrough voltagethat is higher than a peak voltage of a maximum voltage of a voltagesupply.
 6. The surge protection circuit of claim 1, further comprising:a power factor correction module, and wherein the two outputs of therectification module are connected as inputs to the power factorcorrection module.
 7. The surge protection circuit of claim 6, whereinthe discharge device comprises: a discharge resistor; and a Zener diode;and wherein the discharge resistor and Zener diode are connected inseries between the first plate of the capacitor and an output of thepower factor correction module.
 8. The surge protection circuit of claim7, wherein the Zener diode has a breakthrough voltage that is higherthan a difference between the peak voltage of the maximum voltage of avoltage supply and a minimum voltage of the output of the power factorcorrection module.
 9. The surge protection circuit of claim 1, whereinthe capacitor is configured such that it is discharged upon occurrenceof a mains dip event.
 10. A surge protection circuit for a circuitincluding a rectification module, the surge protection circuitcomprising: a first diode; a capacitor; and a discharge device; whereina first terminal of the first diode is connectable to an output of therectification module, a second terminal of the first diode is connectedto a first plate of the capacitor, and a second plate of the capacitoris connectable to the other output of the rectification module; andwherein: the capacitor is configured such that it is consistentlycharged to substantially a peak value of a supply voltage during normaloperation; and the discharge device is connected to the capacitor and isconfigured to discharge the capacitor when the voltage across thecapacitor is in excess of the peak value of the maximum supply voltageand not substantially discharge the capacitor when the voltage acrossthe capacitor is not in excess of the peak value of the supply voltageduring normal operation.
 11. The surge protection circuit of claim 10,wherein the capacitor is configured such that its charge increases whena surge event occurs, and the increased charge discharges to thedischarge device after the surge event.
 12. The surge protection circuitof claim 10, wherein the capacitor is configured such that it is onlysubstantially discharged after a surge event.
 13. The surge protectioncircuit of claim 10, wherein the discharge device comprises: a dischargeresistor; and a Zener diode; wherein the discharge resistor and theZener diode are connected in series between the first plate of thecapacitor and the second plate of the capacitor.
 14. The surgeprotection circuit of claim 13, wherein the Zener diode has abreakthrough voltage that is higher than a peak voltage of a maximumvoltage of a voltage supply.
 15. The surge protection circuit of claim10, further comprising: a power factor correction module, and whereinthe two outputs of the rectification module are connected as inputs tothe power factor correction module.
 16. The surge protection circuit ofclaim 15, wherein the discharge device comprises: a discharge resistor;and a Zener diode; and wherein the discharge resistor and Zener diodeare connected in series between the first plate of the capacitor and anoutput of the power factor correction module.
 17. The surge protectioncircuit of claim 16, wherein the Zener diode has a breakthrough voltagethat is higher than a difference between the peak voltage of the maximumvoltage of a voltage supply and a minimum voltage of the output of thepower factor correction module.
 18. The surge protection circuit ofclaim 10, wherein the capacitor is configured such that it is dischargedupon occurrence of a mains dip event.
 19. A method of operating a surgeprotection circuit for a circuit including a rectification module, thecircuit having a first diode, a second diode, a capacitor and adischarge device, wherein a first terminal of the first diode isconnected to a first input of the rectification module, a first terminalof the second diode is connected to a second input of the rectificationmodule, second terminals of the first and second diodes are connected toa first plate of the capacitor, and a second plate of the capacitor isconnected to one of the outputs of the rectification module, wherein thedischarge device is connected to the capacitor, the method comprising:during normal operation: keeping the capacitor consistently charged tosubstantially a peak value of a supply voltage; and in case of a surgeevent: increasing a charge on the capacitor with current received aspart of the surge event; and discharging the current to the dischargedevice after the surge event.
 20. The method of claim 19, furthercomprising the step of not significantly discharging the capacitorduring normal operation.
 21. A method of operating a surge protectioncircuit for a circuit including a rectification module, the circuithaving a first diode, a capacitor, and a discharge device, wherein afirst terminal of the first diode is connected to an output of therectification module, a second terminal of the first diode is connectedto a first plate of the capacitor, and a second plate of the capacitoris connected to the other output of the rectification module, andwherein the discharge device is connected to the capacitor; the methodcomprising: during normal operation: keeping the capacitor consistentlycharged to substantially a peak value of a supply voltage; and in caseof a surge event: increasing a charge on the capacitor with currentreceived as part of the surge event; and discharging the current to thedischarge device after the surge event.
 22. The method of claim 21,further comprising the step of not significantly discharging thecapacitor during normal operation.